Determining an accurate, continuous level measurement in cement plants
February 13, 2018
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By Joe Incontri, KROHNE, Inc.
A variety of issues affect accurate continuous level measurement in cement plants. The most critical concerns are dust, heavy build-ups, low-reflective media, and uneven surfaces in conical and/or air-injected silos with internal objects. Now, new frequency-modulated continuous-wave (FMCW) radar technology, combined with high signal dynamics, offers a way to accurately and reliably measure product levels. One example of this new technology is KROHNE’s OPTIWAVE 6300 C non-contact radar level. Within its measuring range of 262 feet (80 metres), the device provides a ±1/8 inch (3 millimetre) standard accuracy.
Moreover, due to its small antenna beam angle, tank internals or low reflective media are no longer an issue. To control the production and secure the supply of cement, the level sensor’s measuring signal is transmitted to the control room; there the silo’s level can be continuously monitored. The technology improves the supply inventory of remote cement silos and minimizes inefficient loading and costly delays for the end-user.
Lower operating and installation costs
Installation and operating costs are significantly lower with non-contact radar level technology. The two-wire radar needs less wiring, significantly reducing installation and operating costs.
The level sensor is equipped with Pactware™ software featuring an installation wizard that simplifies configuration. The device software uses dedicated algorithms to adapt each instrument to its specific application, making it particularly suitable for measuring uneven product surfaces. Expensive antenna aiming kits are no longer required. In addition, the drop antenna’s shape and smooth surface made of polypropylene (PP) or polytetrafluoroethylene (PTFE/Teflon), minimizes build-ups. As a result, the three- or six-inch (80/150 mm) antennas need less cleaning and do not require expensive purging systems.
Application in the cement industry
The new level measurement technology is being used in several cement industry applications, including level measurement in cement silos, cement dust level measurement, and fly ash measurement.
Level measurement in a cement silo
A cement silo’s level must be continuously measured to control production and ensure the supply of cement to downstream products. At one cement producer, where cement is stored in a large silo with a holding capacity of several thousand tons, operators found that the level did not decrease evenly as cement was removed from the bottom of the silo. Instead, deep pits formed and large chunks adhered to the silo walls.
Moreover, the combination of a dusty atmosphere and low reflectivity presented issues for the ultrasonic level measurement being used.
The ultrasonic device was then replaced with the new contactless level measurement device. The KROHNE radar level meter, with a Polypropylene (PP) drop antenna with a six-inch nominal size, was placed directly on the silo’s roof opening. Since the antenna was not affected by the angle of repose, it did not have to be aligned. Installation was further helped by the device’s pre-configuration, which only required uploading a few more parameters into the device, including silo height, block distance and cone shape.
The new device required less maintenance due to the lack of moving parts and the antenna’s ellipsoidal and completely-enclosed shape, along with its smooth surface, minimized the need for dust protection, regular flushing, and other periodic cleaning tasks.
Measuring cement dust levels
The technology was also used at a cement plant which uses ball mills to grind clinker. This process generates large volumes of cement dust, which is collected, stored, and then fed back into the production cycle.
The 30-foot-tall (nine-metre) dust storage silos are equipped with air injectors, which fluidize the dust and prevent it from forming up on the silo walls. This simplifies extraction, but the injectors lower the dust’s dielectric constant. Combined with the dust-laden air, the lowered dielectric makes it more difficult to obtain a reliable and accurate level measurement.
Plant operators had previously tested ultrasonic, time-domain reflectometer (TDR), and pulse radar devices, but found that none of these technologies could meet the cement plant’s measurement requirements. To provide the required accuracy and reliability in level measurement, KROHNE installed an OPTIWAVE 6300 C level meter with a six-inch (150 mm) PP drop antenna and a six-inch flange connection.
Fitted to the silo roof, the two wire FMCW non-contact radar continuously measures the dust volume left in the silo. Several months of testing showed the device worked well, and the company equipped nine additional silos with similar measuring devices.
Fly ash level measurement
In another application, a building materials producer previously used an ultrasonic level meter to monitor the amount of fine fly ash powder in a conical 20-foot-wide by 65-foot-tall silo (six by 20 m). However, the company found it difficult to conduct accurate measurements due to the low reflective medium in a dusty environment. The challenging product surface has a low dielectric constant. There was also a tendency for the fly ash to build up on the silo walls and the level radar’s antenna. This led to regular instrument failure, with subsequent process interruption.
The producer sought out a reliable level meter that has an approval for use in hazardous dust areas. They opted for the OPTIWAVE 6300 C level sensor, installing the device with a six-inch (150 mm) drop PP antenna, using a six-inch flange.
The 24-26 gigahertz (GHz) FMCW level meter generates a wave signal that enables the producer to accurately and reliably measure the product level in the dusty fly ash silo. The measuring signal is transmitted by a 4-20mA signal to a control room, where the silo level is constantly monitored.
With the new level measurement technology, inefficient loading and costly delays due to false level indication are no longer an issue. In addition, workers no longer need to climb to the top of the silo to clean the sensor, thus substantially improving worker safety.
Non-contact radar level meters incorporate technology that is not impeded by the dusty environment of cement storage facilities. In addition, accurate and reliable product level measurement is not affected by internal structural objects and build-ups in silos.
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